1. Field of the Invention
The present invention relates to a control device and control method for a vehicle in which an engine (internal combustion engine) and an automatic transmission are installed and, more particularly, to a control device and control method for a vehicle equipped with a torque converter having a lock-up clutch and an automatic transmission.
2. Description of the Related Art
In a vehicle having an engine, a transmission transmits torque and rotational speed, generated by the engine, to driving wheels in accordance with vehicle running states. The transmission includes an automatic transmission that automatically and optimally sets a gear ratio between the engine and the driving wheels.
The automatic transmission installed in a vehicle may be, for example, a planetary gear transmission that uses frictional engagement elements, such as clutches and brakes, and a planetary gear set to set a gear, and a belt-type continuously variable transmission (CVT) that steplessly adjusts the gear ratio.
In a vehicle equipped with a planetary gear automatic transmission, a shift line map that includes shift lines (gear shift lines) for attaining an optimal gear based on the vehicle speed and the accelerator operation amount (or throttle opening degree) is stored in an electronic control unit (ECU), or the like. A target gear is calculated by referring to the shift line map based on the vehicle speed and the accelerator operation amount, and then clutches, brakes and one-way clutches, which serve as frictional engagement elements, are engaged or released in accordance with the target gear to thereby set the gear automatically.
In a belt-type continuously variable transmission, a belt is wound around a primary pulley (input pulley) and a secondary pulley (output pulley), which are provided with a pulley groove (V groove). The width of the pulley groove of one pulley is increased, while at the same time, the width of the pulley groove of the other pulley is reduced, to thereby continuously vary the radii (effective diameters) at which the belt is wound around the respective pulleys. Thus, the belt-type continuously variable transmission steplessly sets a gear ratio.
In vehicles equipped with the above automatic transmissions, a shift lever is provided and is operated by the driver. Operating the shift lever changes the shift position of the automatic transmission, for example, to P position (parking range), R position (reverse running range), N position (neutral range), D position (forward running range), or the like. In addition, in recent years, an automatic transmission having a manual transmission mode (so-called automatic transmission having a sequential mode) have become available. This enables operation of the shift lever to selectively shift the gear of the automatic transmission.
In addition, in the vehicle equipped with the automatic transmission, a torque converter may be arranged in a power transmission path from the engine to the automatic transmission. The torque converter is a hydraulic power transmission and, for example, includes a pump impeller coupled to an engine output shaft (crankshaft); a turbine runner coupled to an input shaft of the automatic transmission; and a stator, provided between the pump impeller and the turbine runner via a one-way clutch. The pump impeller rotates together with the engine output shaft. Then, hydraulic fluid discharged from the pump impeller drives the turbine runner to rotate. Thus, the output torque of the engine is transferred to the input shaft of the automatic transmission.
Furthermore, torque converters that include a lock-up clutch are widely used. The lock-up clutch is engaged (lock-up ON) or released (lock-up OFF) depending on operating conditions to thereby improve fuel consumption rate (hereinafter, called fuel consumption).
Japanese Patent Application Publication No. 2004-263875 (JP-A-2004-263875) and Japanese Patent Application Publication No. 3-182648 (JP-A-3-182648) describe technologies related to controls for a vehicle equipped with a lock-up clutch.
JP-A-2004-263875 describes that during slip control for a lock-up clutch (control for adjusting a transmission torque without completely engaging the lock-up clutch), when an accelerator pedal is depressed, operation of a throttle valve is restricted. Thus, racing of the engine is prevented, and degradation of the lock-up clutch is reduced.
JP-A-3-182648 describes that during engagement of a lock-up clutch, when rotational speed the difference between the rotational speed of an engine drive shaft (engine speed) and the rotational speed of a torque converter driven shaft (turbine speed) is smaller than or equal to a predetermined value, the torque of engine power output is reduced. Thus, a torque shock at the time of lock-up is reduced.
Incidentally, lock-up is one of indispensable manners to transmission efficiency of the automatic transmission; however, engagement of a lock-up clutch that uses wet friction material requires durability of the friction material against heating at the time of engagement. In particular, when power input from the engine is large, rotational speed the difference between the engine speed and the turbine speed of the torque converter increases and, therefore, the friction material generates a large amount of heat when the lock-up clutch engages. Thus, it is necessary to use a plurality of sheets of friction material to ensure an engaged state or to take other measures, with a consequence that the size of the torque converter is increased.
In addition, if it is still difficult to handle the heat at the time of engagement even taking the above measures, the lock-up control is cancelled (lock-up clutch is released), thus maintaining the large rotational speed difference between the engine speed and the turbine speed. When the above large rotational speed difference continues for a relatively long period of time, hydraulic fluid (automatic transmission fluid (ATF)) in the torque converter heats up due to agitation and finally the hydraulic fluid overheats.
Note that JP-A-2004-263875 describes a technology for preventing racing of the engine during slip control on the lock-up clutch. In addition, JP-A-3-182648 describes that in order to reduce shock at the time of lock-up, torque of the engine power is reduced when the lock-up clutch is engaged. However, neither JP-A-2004-263875 and JP-A-3-182648 suggest anything about ensuring of the durability of the friction material when the lock-up clutch is engaged. Thus, the technologies described in JP-A-2004-263875 and JP-A-3-182648, provides no solution for the above problem.